Patent Application
20140293251
1. Field
Embodiments of the present invention relate to a projection system, a lithographic apparatus, a method of projecting a beam of radiation onto a target and a method for manufacturing a device.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”—direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, a beam of radiation may be patterned by a patterning device which is then projected onto the substrate by a projection system. This may transfer the pattern to the substrate. It will be appreciated that there is a continual drive to improve the performance of lithography apparatus. Consequently, the requirements for the accuracy of performance of the components within a lithography apparatus correspondingly are continually becoming stricter. In the case of a projection system, one measure of the performance of the projection system is the accuracy with which a patterned beam of radiation may be projected onto a substrate. Any deviation in the position of the patterned beam of radiation may result in errors of the pattern to be formed on the substrate, for example, overlay errors, in which one part of a pattern is not correctly positioned relative to another part of a pattern, focus errors and contrast errors.
In order to minimize errors introduced by the projection system, it is necessary to ensure that optical elements within the projection system that are used to direct the patterned beam of radiation are accurately positioned. Therefore, it has previously been known to provide a stiff frame to which each of the optical elements is mounted and to adjust the position of each of the optical elements relative to the frame in order to position correctly the optical elements.
However, even with such a system, small errors may be introduced. With previously known systems, such small errors were not significantly problematic. However, with the continual drive to improve the performance of lithography apparatus, it is desirable to at least reduce all possible sources of error.
Given the foregoing, what is needed is a projection system, for example, for use within a lithography apparatus, having improved performance.
According to an aspect of the invention, there is provided a projection system, configured to project a beam of radiation. The projection system includes a frame configured to support at least one optical element that is used to direct at least a part of the beam of radiation, a sensor system configured to measure at least one parameter that relates to a displacement of the frame generated by forces applied to the frame during use of the projection system, and a control system configured to determine an expected deviation of the position of the beam of radiation projected by the projection system that is caused by the displacement of the frame using the measurements of the sensor system.
According to an aspect of the invention, there is provided a lithographic projection apparatus that uses a projection system as disclosed above to project a patterned beam onto a substrate.
According to an aspect of the invention, there is provided a method of projecting a beam of radiation onto a target. The method includes directing the beam of radiation using at least one optical element that is supported by a frame, measuring at least one parameter that relates to a displacement of the frame generated by forces applied to the frame while projecting the beam of radiation onto the target, and determining an expected deviation of the position of the beam of radiation that is caused by the displacement of the frame using said measured at least one parameter.
According to an aspect of the invention, there is provided a device manufacturing method comprising projecting a patterned beam of radiation onto a substrate, using a method of projecting a beam of radiation onto a substrate as disclosed above.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:
a and 2b depict a problem that may reduce the performance of a projection system.
an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or EUV radiation.
a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters;
a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and
a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of substrate W.
The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.
The support structure supports, i.e., bears the weight of, the patterning device.
It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.
The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.
As here depicted, the apparatus is of a reflective type (e.g., employing a reflective mask). Alternatively, the apparatus may be of a transmissive type (e.g., employing a transmissive mask).
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.
Referring to
Illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as a-outer and a-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, illuminator IL may include various other components, such as an integrator IN and a condenser CO. Illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.
Radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed mask MA, radiation beam B passes through projection system PS, which focuses the beam onto a target portion C of substrate W. With the aid of second positioner PW and position sensor IF2 (e.g., an interferometric device, linear encoder or capacitive sensor), substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of radiation beam B. Similarly, first positioner PM and another position sensor IF1 can be used to accurately position mask MA with respect to the path of radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of first positioner PM. Similarly, movement of substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of second positioner PW. In the case of a stepper (as opposed to a scanner) mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on mask MA, the mask alignment marks may be located between the dies.
The depicted apparatus could be used in at least one of the following modes:
1. In step mode, mask table MT and substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). Substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of target portion C imaged in a single static exposure.
2. In scan mode, mask table MT and substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of substrate table WT relative to mask table MT may be determined by the (de-)magnification and image reversal characteristics of projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.
3. In another mode, mask table MT is kept essentially stationary holding a programmable patterning device, and substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.
Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.
As explained above, and as depicted in
It will be appreciated that this problem may be reduced, for example, by increasing the stiffness of the projection system frame 10 such that the external forces acting on the projection system result in smaller deformations of the frame 10 and therefore smaller deviations of the beam of radiation projected by the projection system. However, this may result in an increase in the weight and/or volume of the projection system, which may be undesirable.
A particular problem with the deviation of the position of the projection beam of radiation projected by the projection system caused by deformation of the projection system frame 10 is that it is difficult to measure directly the deviation of the projection beam of radiation during production, namely whilst projecting beams of radiation onto substrates in order to form devices.
Therefore, according to an embodiment of the present invention, a system such as that schematically depicted in
The expected deviation determined by control system 30 of beam of radiation B that is, for example, projected onto substrate W, may be used to ameliorate the effects of the deviation caused by the deformation.
For example, as explained in more detail below, one or more corrections may be made based on the expected deviation of beam of radiation B. These corrections compensate for the expected deviation of beam of radiation B from an intended location such that beam of radiation B is more accurately projected onto the desired location of substrate W.
Alternatively, or additionally, the expected deviation may be recorded. This may provide data that is useful, even if no steps are taken to compensate for the expected deviation. For example, by monitoring the expected deviation that is determined by the control system 30, operation of the projection system may continue while the expected deviation is within an acceptable limit but may be suspended if the expected deviation exceeds that limit. Likewise, monitoring of the expected deviation may be used to schedule maintenance operations of the projection system, for example in order to make corrections to the system before the expected deviation exceeds a tolerated extent. Similarly, monitoring the expected deviation of the location of projection beam B from its desired target location on substrate W may be collated for each substrate and/or each device being formed on a substrate, such that the quality of formation of the devices may be graded.
Control system 30 may include a model 31, such as a mathematical model that represents the projection system. In particular, model 31 may relate the parameters measured by sensor system 20 to the deformations of frame 10. In turn, model 31 may relate the deformations of frame 10 to the expected deviation of beam of radiation B projected by the projection system. Accordingly, control system 30 may use a processor 32 and model 31 in order to determine the expected deviation of beam of radiation B projected by the projection system, based on the measurement data from sensor system 20. Processor 32 may then respond in a desired fashion, for example taking steps necessary to compensate for the expected deviation, as explained in more detail below.
Alternatively, or additionally, control system 30 may include a memory 33 containing calibration data. The calibration data may directly relate the measurement data from sensor system 20 to the expected deviation of beam of radiation B projected by the projection system.
For example, the calibration data stored in memory 33 may be generated by performing a series of tests before the projection system is used in, for example, the manufacture of devices. Accordingly, a series of external forces may be applied to the projection system. For each loading condition, measurements may be taken and recorded by the sensor system. At the same time, direct measurements of the deviation of beam of radiation B projected by the projection system may be made. This data may then be used as the calibration data.
It will be appreciated that processor 32 within control system 30 may be configured such that processor 32 can interpolate between sets of calibration data. This may reduce the amount of calibration data that may need to be stored in memory 33. Such an arrangement may be faster to operate than a system including a model 31 such as that discussed above. However, the accuracy of the determination of the expected deviation of beam of radiation B may be limited, for example, by the amount of calibration data stored in memory 33.
In a particular embodiment of a projection system, such that depicted in
The one or more accelerometers 21 may be configured to measure the acceleration of frame 10 of the projection system in, for example, all six degrees of freedom. However, it will be appreciated that this may not be necessary in order to improve the performance of the projection system. Accordingly, the one or more accelerometers 21 may measure the acceleration of the frame 10 in a more limited set of degrees of freedom.
It should also be appreciated that it may be sufficient to configure the one or more accelerometers 21 to monitor the acceleration of a single part of frame 10. Alternatively, however, the accuracy of the determination of the expected deviation of beam of radiation B projected by the projection system may be improved by configuring the one or more accelerometers 21 such that the acceleration of more than one part of the frame 10 is separately monitored.
The measured acceleration of one or more parts of frame 10 of the projection system will be related to the external forces applied to frame 10 and therefore to the deformations that will be induced in frame 10 by those external forces. Accordingly, control system 30 may determine the external forces applied to the projection system based on the measurement data from the one or more accelerometers 21. Controller 30 may then use that force data to determine the expected deviation of beam of radiation B as described above. Such an arrangement may be particularly beneficial for a projection system to be used in a lithographic apparatus in which extreme ultraviolet (EUV) radiation is used to image a pattern onto a substrate. In such apparatus, the projection system is typically arranged in an evacuated chamber in order to minimize absorption of the EUV beam of radiation by gas within the system. In such an arrangement, the only external forces that may be applied to frame 10 of the projection system are transmitted through the mounting points by which the projection system is mounted to the remainder of the lithographic apparatus. For example, other external forces, such as acoustic disturbances transmitted through the gas surrounding the projection system may be eliminated or reduced to an insignificant level. By reducing the possible mechanisms for transmitting external forces to the projection system, it may be relatively straightforward to determine accurately the forces exerted on the projection system that produce the accelerations measured by the one or more accelerometers 21. Accordingly, accurate determinations of the expected deviation of beam of radiation B may be based on the data from the one or more accelerometers 21.
Alternatively or additionally, as depicted in
For example, mounts 15 may be used to mount the projection system to a reference frame 16 within a lithographic apparatus. In particular, sensor system 20 may be arranged such that each of mounts 15 that supports frame 10 of the projection system may be associated with a force sensor 22. Such a system may provide direct measurement of substantially all of the external forces applied to the projection system or, at least, the most significant forces, namely those that result in the largest deformations of frame 10. Accordingly, from these measures, control system 30 may determine the expected deviation of beam of radiation B projected by the projection system with considerable accuracy.
It should be appreciated that, in an embodiment, force sensors 22 may be an integral part of mounts 15. This may, in particular, be the case if mounts 15 include actuators that may be used to adjust the position of the projection system. In such an arrangement, force sensors 22 may in any case be provided in order to control the actuators. Force sensors that are not integral to mounts 15 may alternatively or additionally be used.
Alternatively or additionally, as depicted in
Alternatively or additionally, as depicted in
It will be appreciated that any combination of the above described sensors may be combined together to form sensor system 20. Likewise, other sensors may be used in order to provide measurements of alternative or additional parameters that are related to the deformation of frame 10 of the projection system.
As discussed above, control system 30 may be arranged in order to use the expected deviation of beam of radiation B from its intended location that is determined from the sensor system data in order to compensate for the deviation.
For example, as shown in
Alternatively or additionally, the position of the projection system relative to an apparatus to which it is mounted, such as a lithographic apparatus, may be controlled by an actuator system 42, as depicted in
Alternatively or additionally, as depicted in
As discussed above, a projection system of an embodiment of the present invention may be utilized within a lithographic apparatus. Within such a lithographic apparatus, a support MT may be provided to support patterning device MA that imparts a pattern to beam of radiation B. Beam of radiation B may then be projected, using a projection system according to an embodiment of the present invention, onto a substrate W held on a substrate table WT.
In such an arrangement, control system 30 may alternatively or additionally be configured to control an actuator system PM that controls the position of patterning device MA in order to compensate for the expected deviation of beam of radiation B projected onto the substrate. In particular, movement of patterning device MA relative to beam of radiation B that is incident thereon may adjust the position of the pattern within the cross-section of the beam or radiation. Control system 30 may therefore adjust the position of the patterning device PM such that, although beam of radiation B may not be projected onto substrate W at precisely the desired location, the pattern that is projected onto the substrate is more accurately positioned relative to its desired location on the substrate.
Alternatively or additionally, control system 30 may be arranged to control an actuator system PW that is provided to control the position of substrate W in order to compensate for the expected deviation of beam of radiation B projected by the projection system onto substrate W. Accordingly, although beam of radiation B may be deviated from its intended position relative to the projection system, it is more accurately positioned relative to its desired location on substrate W.
It should be appreciated that control system 30 may be configured to use any combination of the arrangements discussed above for compensating for the expected deviation of beam of radiation B that is determined based on the measurements from sensor system 20.
As explained above, and as depicted in
Therefore, according to an embodiment of the present invention, a system such as that schematically depicted in
The expected deviation determined by control system 30 of the beam of radiation that is, for example, projected onto substrate W, may be used to ameliorate the effects of the displacement of the frame 10.
For example, as explained in more detail below, one or more corrections may be made based on the expected deviation of the beam of radiation. These corrections compensate for the expected deviation of the beam of radiation from an intended location such that beam of radiation is more accurately projected onto the desired location of substrate W.
Alternatively, or additionally, the expected displacement may be recorded. This may provide data that is useful, even if no steps are taken to compensate for the expected displacement. For example, by monitoring the expected displacement that is determined by the control system 30, operation of the projection system may continue while the expected displacement is within an acceptable limit but may be suspended if the expected displacement exceeds that limit. Likewise, monitoring of the expected displacement may be used to schedule maintenance operations of the projection system, for example in order to make corrections to the system before the expected displacement exceeds a tolerated extent. Similarly, monitoring the expected displacement of the location of projection beam from its desired target location on substrate W may be collated for each substrate and/or each device being formed on a substrate, such that the quality of formation of the devices may be graded.
Control system 30 may include a model 31 (see
Alternatively, or additionally, control system 30 may include a memory containing calibration data. The calibration data may directly relate the measurement data from sensor system 20 to the expected displacement of the beam of radiation projected by the projection system.
For example, the calibration data stored in the memory may be generated by performing a series of tests before the projection system is used in, for example, the manufacture of devices. Accordingly, a series of external forces may be applied to the projection system. For each loading condition, measurements may be taken and recorded by the sensor system. At the same time, direct measurements of the displacement of the beam of radiation projected by the projection system may be made. These measurements may be made by a Transmission Image Sensor (e.g., such as U.S. Pat. No. 7,675,605 assigned to ASML Netherlands B.V.) for measuring a position of an image produced with the projection system. This data may then be used as the calibration data.
In a particular embodiment of a projection system, such that depicted in
The one or more accelerometers 21 may be configured to measure the acceleration of the frame 10 of the projection system in, for example, all six degrees of freedom. However, it will be appreciated that this may not be necessary in order to improve the performance of the projection system. Accordingly, the one or more accelerometers 21 may measure the acceleration of the frame 10 in a more limited set of degrees of freedom.
The measured acceleration of one or more parts of frame 10 of the projection system will be related to the acceleration of the optical elements 11 and therefore to the displacements that will be induced in optical element 11 by those external forces. Accordingly, control system 30 may determine the displacement of the optical elements based on the measurement data from the one or more accelerometers 21. Controller 30 may then use that data to determine the expected displacement of the beam of radiation as described above.
The model in
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that embodiments of the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “lens,” where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.
While specific embodiments of the invention have been described above, it will be appreciated that embodiments of the invention may be practiced otherwise than as described. For example, embodiments of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein.
The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.
| Number | Date | Country | |
|---|---|---|---|
| 61089820 | Aug 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13002840 | Apr 2011 | US |
| Child | 14305110 | US |
